Vent control for processing containers



VENT CONTROL FOR PROCESSING CONTAINERS Filed April 25, 1947 /96INVENTOR.

'RMDW Patented not. 21, 1950 VENT CONTROL FOR PROCESSING CONTAINERSRobert D. Cowherd, West Terre Haute, Ind., as-

signor to The Bristol Company, Waterbury, Conn., a corporation ofConnecticut Application April 25, 1947, Serial No. 743,925

24 Claims.

This invention relates to the automatic control of the venting of fluidfrom a processing container or a reaction vessel, particularly a pulpdigester, in which there is evolved a vapour atmosphereirom amulti-component liquid.

In the manufacture of wood pulp it is customary to. charge a closedvessel known as a digester with quantity of wood chips and suitablechemicals and then to set up a reaction by the admission of steam orother heating agent. In the course of this process certain gases aregiven off; and it is usual to vent these from the top of the digester atsuch a rate as tomaintainsaturated steam pressure and temperature valueswithin the digester. During the chemical reaction, the caustic'sodasaponifies the rosin to form a rosin soap. The reduced surface tensionof the solution due to this soap imparts a tendency toward foaming, withthe result that some of the solution may be drawn off with the gas beingvented from the top of the digester. If extreme foaming or pull-over ispresent, the chemical content of the digester will be depleted to suchan extent as to interfere with the conversion of the wood to pulp.

Automatic control of the rate of venting digester gases is not new. Itis customary to equip digesters with instrumentalities which willregulate the rate of gas-off in accordance with the amount of gasesother than steam present in the venting medium. The object of such acontrol is to maintain these reaction gases at a minimum such that thetemperature of the reaction will rise with the pressure in a steam tablerelationship. Another manner of expressing the above is: The rate ofventing a digester must be such that the partial pressure of gases otherthan steam is held to a minimum value. A common means for determiningpartial pressure of such gaseous mixtures involves the use of apartially filled water thermal system arranged to respond to the thermalpotential of the gases, and differentially combined with a pressuremeasurement taken at the same point. The use of such a combination isrecognized in the papermaking industry and in other fields, and may beeffected by any one of a number of well known methods, one of which isset forth in U. S.

Letters-Patent No. 1,446,097 granted February 20, 1923 to E. Lindbom andC. W. Burroughs. If the gaseous phase of the digester contents issaturated steam, the pressures in the two syst ms will be identical. Ifreaction gases other than water vapour are present, the decreasedthermal potential due to their partial pressure will cause a lowerpressure in the water-filled thermal system than the actual pressure inthe reaction vessel. The difference between these pressures is a measureof the partial pressure of the gases other thanwater vapour. Suchdevices, however, take no cognizance of the condition of the fluid beingvented, and will vent the digesting liquor as readily as-the reactiongassteam mixture.

g It is recognized also, that there are available various phasediscriminating devices, lying in the general category of steam traps,operating for the most part either on a gravimetric principle or on abasic of relative temperature of a liquid and its vapour; but, as theformer require intricate weighing mechanisms and are inherently slow intheir action, and as the factor of actual temperature is not necessarilyan element in the desired discrimination, such devices are not suited tothe objects of the invention.

In my Patent No. 2,411,986, issued December 3, 1946, I have disclosed amethod and apparatus for controlling the venting of the products of thereaction, predicated upon the difference between the energy transmittingproperties of the liquid, as distinguished from the gaseous componentsof said products. In the embodiments disclosed in said application,there is applied to the vent pipe from the digester a force tending tocause a flow of energy through the contents of said pip and the amountof energy passing through a liquid component of the effluent, asdistinguished from that passing through a gaseous component, is utilizedto cut off the venting. So long as no liquid appears in the vent pipe,the reaction products are vented freelyi-but, upon the appearance ofsaid liquid in said pipe, the change in the amount of energytransmittedtherethrough causes the venting to be terminated.

My present invention achieves effective regulation of the venting,without reliance upon the different amounts of energy transmitted by theliquid and the gaseous phases of the efliuent, and without dependenceupon the previously mentioned reaction gas partial pressure control. Itis based on my discovery that the electrical conductivity (orresistivity) of the vapour atmosphere evolved by the digester may beemployed to regulate the venting of the gases, so as to maintain thedesired conditions of saturated steam temperatures and pressures withinthe digester. I have further found that the electrical resistivity ofsaid vapour phase is a measure of the partial pressure of thenon-aqueous vapours in the vapQ ll atmosphere evolved by thereactdigesters recognizes two more or less distinct methods, namely, thedirect and the indirect. Each has its advantages and its disadvantages.In the direct method, cooking is accomplished by admitting steamdirectly to the digester and allowing it to enter into combination withthe materials therein. In the indirect method, the heat necessary forthe reaction is communicated to the contents of the digester by suitableheat exchangers through which the cooking liquor is circulated. Steam isthe usual heat source.

If relief of the gaseous contents of a digester can be effected withoutpulling over the liquor, operation of the digester is said to be stable.If such relief cannot take place without the loss of liquor theoperation is said to be unstable. Between these extremes there may existvarious degrees of stability. The hereinbefore discussed distinctmethods of cooking pulp are quite different in their stabilitycharacteristics; and in any one installation the stability may varyconsiderably between successive cooks. While the principles of thepresent invention are suited to the control of digesters operatingaccording to either indirect or direct cooking cycles, or anycombination of the two, the advantage is especially apparent inobtaining optimum performance from digesters subject to unstableoperation or to the variation between the unstable and stable conditionsoccuring in the course of a normal cook.

In a process of direct heated digesters, the mixing of steam with thecontents of the reaction vessel, causes the process to be subject tounstable venting of reaction gases. The liquid level within the digesteris increased due to the condensation of the directly admitted steam. Therising level decreases the foam distance from the vent line so thatpull-over of the foam and liquid into the vent line is more likely tooccur during the venting of the reaction gases. Furthermore, the erraticchanneling of steam within the contents of the digester gives rise topockets of liquor of a higher temperature than that of other portions ofthe digester. When these pockets of greater heat content equalize, dueto excessive internal pressures, violent turbulence of the digestercontents may occur, with a resultant pull-over of liquor into the ventline. It will thus be seen that the direct steamed digester is likely tobe inherently unstable in its venting characteristics.

The indirectly heated digester has neither of the above-mentionedfaults. The use of heat exchangers eliminates any increase in the liquidcontent of the digester; and the use of circulating pumps minimizesunequal heat distribution. Thus, in the indirectly heated digester, thereaction gases may be freely vented without excessive pull-over. Due tothis stability, the gases may be vented at such a rate as to insure thatthe amount of gases other than steam is a minimum. Under suchconditions, the gases will tend to assume saturated steam conditions,and the temperature of the vessel will approach that of the steamequivalent to the digester pressure.

If 9. directly heated digester were to be vented at the same rate as anindirectly heated digester, liquid pull-over would be abnormal. It is,therefore, necessary to establish a lower continuous venting rate withno pull-over of liquor, rather than a higher rate with liquor pull-over.To effect this result the accent must be removed from the establishmentof saturated steam conditions in the vented gases and placed upon theability of the digester to vent at all. Good practice indicates thatoven-all operation is much better if the digester is allowed to gascontinuously at a low rate rather than intermittently at a high rate.

In the control system disclosed in my Patent No. 2,411,986, it will beobserved, that a decrease in the resistivity of the eflluent from thedigester indicates the pulling over of liquor into the vent line. Thismay be either in the liquid or the foam condition. At this time, thevent line is closed and steam admitted in such a manner that it purgesthe electrode of any conducting medium in its path to the digester.After a timed interval, the steam is shut off and the vent opened. Ifliquor still is in the digester effluent, the process of steam blow-backis repeated. When the effluent is in a gaseous state, the vent is openedand remains so until another pull-over of liquor occurs.

I have now discovered that, not only is the resistivity of the eilluentaffected by a change in its liquid-gas phase, but that the r:sistivityof the gaseous phase is directly related to the partial pressure ofcomponent gases other than steam; and operating experience withinstallations embodying the principle has shown that the electrodesystem used to differentiate between the gaseous and liquid phases mayperform not only this function but also that of analyzing the gaseousphase. The electrical resistivity of water vapour in the form of steamis high. As the partial pressure of steam in the mixture of steam andgases other than steam decreases the resistivity of the mixturedecreases at a relatively high rate. The electrical resistivity of thegaseous effluent can therefore be utilized as a highly sensitive meansfor determining the partial pressures of steam and gases other thansteam. The sensitivity and speed of response of this method are such asto show that the pu l-over of liquor is preceded by a rapid change inthe partial pressure of the non-aqueous vapors, thus enabling theapproach of the liquor to the venting system to be anticipated, andmaking possible remedial measures in the form of a decrease in thegas-off rate sufficiently early to prevent the liquor from reaching theventing sys- The sensitive partial pressure change response of theelectrical measuring method may be utilized in the control of a digesterin two separate and distinct methods of regulation based upon thestability of the process; and a combination of these two may beincorporated into a single control function on either type of digestersystem.

As hereinbefore pointed out, an indirectly heated digestion process isinherent y stable. It is therefore possible to control the steam partialpressure in the following manner: As the partial pressure of the steamdecreases the electrical resistivity of the gas stream also decreases.In

.76 O er to attain saturated conditions in the gas nating the gasesother than steam, so that the steam partial pressure will rise. As thepartial pressure increases the resistivity of the gas will alsoincrease, indicating that, since the partial pressure of the steam inthe vent gases is approaching conditions corresponding to saturation,the valve should be closed. Neither of these functions is of anon-and-ofl character. The control setting should be placed at aresistivity value as close to saturated steam conditions as willeconomically operate the process. As this critical value is attained,the vent valve will be c osed. Any decrease in gas resistivity away fromthe set point will cause the valve to be opened to a correspondingextent, so that the control function becomes one based on deviation froma saturated or near-saturated condition.

The directly heated digestion process, on the other hand, and aspreviously pointed out, is inherently unstable: and the tendency is tovent liquor rather than gas. If a control function as just describedwere a plied to the venting of such a process, any opening of thegas-off valve, with the corresponding decrease in resistivity of theeilluent, would induce further pull-over. It has been found, however,that where an unstable process is involved a decrease in resistivity isa prior index of pul -over of liquor into the vent line, and also that adecrease in the rate of venting will introduce a stabilizing influence:and the present inventon is directed in part to the utilization of thesediscoveries. By closing the vent valve upon a decrease, and opening itupon an increase, of the electrical resistivity of the gas stream, withreference to that value corresponding to saturated steam conditions, thre is established a gas-oil rate proportionate to digester stability.

It has been found that, due apparently to non-uniform heat distribution,the first art of the cook in either the direct or the indirect method ofdigester operation is likely to be of an unstable charact r. Therefore,without rcspect to the characteristic of the overa l process. it becomesdesirable during this period to effect control on a basis oiinstability, and, as st ady conditions are attained, to transfer to thattype of control best suited to the system. As the temperature of theeiiluent when operating under stable conditions is materiallv higherthan that wh n the operation is unstable, it becomes possible to utilizethat temperature as a basis for transition; and, having empiricallydetermincd a suitable value corresponding to any selected set ofconditions and materials, to cause the control function to be shiftedfrom unstable to stab e performance when that temoerta re is attained.Thus, during the unstable initial stages of the cook, accent will beplaced "non the maximum rate of gas-oil for the stab lity present in theprocess, and after heat distribution has eliminated or decreased thprobabilit of unstable o eration, the accent will be placed upon theability to obtain saturated or n arsaturated steam conditions in thevent gases.

In the drawings:

Fig. l is a diagrammatic representation of a pulp-dige ter showing theinvention applied to the control of relief of internal pressure byregulating the outflow or products of the reaction.

Fig. 2 is a diagram showing in detail and to an enlarged scale a certainelement of the apparatus generally indicated in Fig. 1.

stream, the vent valve must be opened. elimi- Referring now to thedrawings:

The numeral ll designates a pulp-digester into which mixture of woodchips and suitable chemicals may be charged through an opening in thetop which is afterwards sealed by a cover plate II. A conduit i2provides for the admission of a suitably regulated flow of steam to thelower part of the digester, and a conduit it connected to the bottom ofthe digester provides means whereby the contents thereof may bedischarged to a blow tank at the conclusion of each cook. A conduit llconnected near the top of the digester provides for the venting ofgaseous products of the internal reaction to the atmosphere, or to areceptacle not shown in the drawings whereby certain valuableconstituents of said products may be salvaged. Such venting may beeffected through either of two diaphragm valves II and i6, connected ina branched section of the conduit H, which afterwards reunites toprovide a connection to said receptacle, or to the atmosphere, as thecase may be. The valve II is of the reverse-acting type, its actionbeing such that increase of pressure applied to its diaphragm top tendsto increase the degree of opening of the valve, and vice versa. Thevalve i0 is of the "direct-acting" type, its action being such thatincrease of pressure in the diaphragm top tends to close the valve andvice versa.

A pipe I! provides for the admission of steam or other suitable agent tothe conduit H, whereby the part of the conduit in immediate proximity tothe digester may be flushed out and cleared of accumulated liquid orsemi-liquid material, and any tendency toward foamin of the digestercontents suppressed. A diaphragm valve ii of the reverse-acting type isinserted in the pipe I! and has its operating element placed incommunication with a conduit I9 whereby air pressure in said conduitwill cause said valve to be opened.

A controlling instrument 20, responsive to temperature changes asdetermined by a suitable bulb 2i inserted in the conduit H, is providedwith a. defiectable index or pointer 22 adapted by its position withrespect to a graduated scale 23 to provide a measure of the temperatureto which the bulb 2! is exposed. Juxtaposed to the pointer 22, andpreferably coaxially adjustable with respect thereto. is an arm 24carrying electric contact elements 25, adapted to be engaged andinterconnected by a further contact element 26 carried by the index orpointer 22. The arm 24 being adjustable, the temperature at which thecontacts 25 will be interconnected may be set at will to any desiredvalue of temperature as indicated by the position of the arm 24 withrespect to the scale 23.

Inserted in the conduit ll, so as to provide intimate electric contactwith the contents thereof, are two electrodes 30 and 3i, one at least ofwhich must be thoroughly insulated from said conduit. A suitable bridgeand amplifier unit 32 is provided with input terminals 33 and outputterminals 1, so interrelated bymeans of a suitable resistance-responsiveelectrical network that variations in electrical resistance between theterminals 33 will be reproduced in corre ponding variations in anelectrical potential appearins at the terminals 34. The electricalnetwork within the unit 32 may consist of a transformer energized from asuitable alternating current source, having its primary winding inseries with the terminals 33 and its secondary winding connected to theterminals 34 through a rectifier network, as shown in my Patent No.2,411,986, or it may be or a more elaborate and reflned nature,including any one of a large variety of suitable circuits well known inthe art of resistance measurement, and forming no essential part of thepresent invention.

A controlling instrument 35 includes an index or pointer 36, deflectablein response to changes in the electric potential developed at theterminals 34 of the unit 32', and, by its position with respect to agraduated scale 31', adapted to provide a measure of said potential, andthus of the resistance between the electrodes 39 and 3i in the conduitl4. Positioned within the instrument 35 are electrical contact elements38 adapted to be engaged and interconnected by a further contact element39 carried by the index or pointer 38. The contacts 38 are so positionedas to be engaged when said pointer is at a position substantially at thebottom of its scale, corresponding to a relatively low resistanceexisting between the electrodes 30 and II.

Included in the instrument I is a pneumatic control element 40, whichmay expediently be of the type fully set forth and disclosed in U. S.Letters Patent No. 1,880,247, granted October 4 1932 to Griggs andMabey. The unit 40 has a movable arm 4| connected by means of a link 42to the pointer 35, so that motion of said pointer will correspondinglydeflect said arm whereby to control the pressure of air from a conduit43 connected to source 44, to a variablepressure conduit 45. Therelationship between the control element 40 and the index or pointer 36is made such that a deflection of the latter in response to an increasein measured resistance will cause the pressure of air in the conduit 45to be increased, and a deflection of said pointer toward the bottom. orleft hand end, of the scale in response to a lowering of said resistancewill cause said air pressure to be lowered.

The diaphragm element of the reverse-acting valve I5 is connected to theconduit 45 by means of a conduit 46 having therein a reverse-actingsolenoid-actuated valve 41 of the type which will be closed when itselectrical winding is energized and open when said winding isdeenergized. The diaphragm element of the directacting valve I6 isconnected through a conduit 48 alternatively to the conduit 45 or to thesupply conduit 43 by means of a solenoid-actuated three-way valve 48arranged that when electrically energized it provides a directconnection between conduits 48 and 45 and when deenergized it provides adirect connection between conduits 48 and 43. The conduit l9,communicating with the diaphragm element of valve I8, is connected tothe supply conduit through a direct-acting solenoid valve Bil, whereby,when the solenoid is energized, air pressure will be directly applied tothe valve ii to open its parts, and when said solenoid is deenergizedthe valve l8 will be closed.

While, in practice the bodies of the valves 41 and 59 would probably beof the three-way type, allowing the conduits 46 and I5 respectively tobe vented to the atmosphere, whereby quickly to deflate the diaphragmsof the valves I5 and 18 when said solenoid valves are closed, thesedetails, for purposes of clarity are omitted from the presentspecification, as being well understood by those versed in the art ofpneumatic control and forming no essential part of this invention.

An electrical relay 5| having two normally open contacts Bio and 5": anda normally closed contact Blc, provides for actuation of the solenoidvalves 41, 49 and 58 in a manner presently to be described. A combinedtiming and interrupter unit 52, shown diagrammatically in detail in Fig.2, serves to introduce such time elements as are necessary to thecontrol cycle. While the function of the timing element shown in Fig. 2may be satisfactorily eifected by means of a timer such as that fullyset forth and described in U. 8. Letters Patent No. 2,175,865 granted toC. L. Anderson October 10, 1939, the device is here, for purposes ofclarity, shown to a more conventionalized form of reprsentation. A relayassembly comprises a solenoid 53 adapted to actuate a plunger 54carrying a rod or stem 55 to which are afllxed three contact elements51, 58 and 59. Said contact elements are adapted to engage respectivelythree corresponding pairs of contact points 51a, 58a and 59a, allinsulatedly mounted upon a movable plate 60. Said plate is provided withslotted openings GI and is attached by means of suitable screws or boltspassing therethrough to the base of the unit 52, in such a manner thatthe plate may be moved through a limited range in the same sense as thenormal direction of operation of the stem 55. The arrangement of thecontact elements is such that, when the plate is constrained in itspositionnearest to the solenoid 53 (by means presently to be described)and said solenoid deenergized, the contacts 51a and 59a are normallyopen and the contact 58a normally closed. Upon energization of thesolenoid 53, the condition of said contacts is reversed, contacts 51aand 59a being closed and 58a opened. The plate 60 is provided with anabutment portion 55, and the stem 55 carries a collar 66 juxtaposedthereto. A compression spring 61 interposed between said abutment andcollar tends upon energization of the solenoid 53 to force the plate 60and contact points carried thereby through a limited distance in thesame direction as the motion of the stem 55 and the contact elementscarried thereby.

The movable plate 60 is provided with a detent member 68 integraltherewith and having a perpendicularly disposed surface adapted tocooperate with a similar and opposed surface on a latch member 69deflectable through a limited angle about an axis perpendicular to thebase plate 01' the unit 52 and also to the direction of travel of theplate 60, whereby said plate will be restrained from motion in responseto the influence of spring 61. At least one of said members is providedwith an inclined "rear" surface whereby the plate 60 may freely assumeits normal position, the latch yielding to allow the detent to passuntil the position of engagement of the cooperatin perp ndi ularsurfaces is reached. The latch member 69 is urged toward its position ofengagement with the detent 68 by means of a weight or equivalent l0tending to rotate said latch m mber about its axis of deflection. Astationary stop ll limits the excursion of said latch member in thedirection of its engaging position, and prevents its fouling the movableplate 60.

Mounted for limited angular rotation about the same axis as the latchmember 69 is a worm wheel 15 having through its peripheral portion anumber of openings in any one of which may be selectively secured amovable pin 16 adapted upon rotation of said worm wheel in acounter-cinch wise sen e to engag the latch member 69, deflecting itthrough a slight angl w ere y to release the detent 68. A stop member 11fixed to the rim of the worm wheel 15 is adapted to engage a stationaryabutment 18, whereby to provide a urged by spring, weight, or equivalentmeans not' shown in the drawing. An electric motor 50 is adapted torotate a shaft i carrying a worm 82 and having one end journalled in amovable bearing 53 carried by the stem 55, whereby when the solenoid 53is energized the worm 52 will be brought into operative engagement withthe worm turned to its position of repose with the stop I1 engaging theabutment I0.

The interrupter portion of the unit 52 includes an electric motor 55connected to drive a cam member 55 which engages a movable arm 51carrying a contact element 55 adapted to provide electrical connectionbetween two stationary contact points 89, the arrangement being suchthat when the motor 55 is in operation electrical connection betweensaid contact points will be periodically made and interrupted atintervals of the order of several seconds.

The interrupter and timing unit 52 is provided with live terminals, 90,9 I, 32, 93, and 95, whereby connection may be made between externalcircuits and the several electrical elements embodied in the unit 52.said terminals and electrical elements in the unit 52 are as follows:The contact points 51a, 53a and 55a carried by the plate 50 have eachone side connected by suitable flexible leads to a common conductor 95which in turn is connected to the terminal 92. The free member of thecontact points 51a is connected by a flexible lead to the terminal 93,and that of the contact points 58a to the terminal 94. The free memberof the contact points 59a is connected by a flexible lead to a conductor96, to which also is connected one terminal of the solenoid 53, oneterminal of the motor 80, and one of the contact points 53. The freemember of the contact points 50 is connected by means of a conductor 51to the terminal 90, and to this terminal also is connected one side ofthe interrupter motor 85. The terminal Si is connected to a conductor 35which is common to the free terminals of the motor 85, the motor 80, andthe solenoid 53, respectively.

Following are the electrical connections by which the combinedinterrupter and timing unit 52 is operatively associated with the otherelements of the system; A source of electric supply is represented bytwo conductors I00 and "I.

The interconnections between i 0 lie are connected by a conductor I01 tothat of the solenoid 41.

Before attempting to consider the operation of the control system as awhole, it will be well to study the functioning of the several elementsincorporated within the combined timing and interrupte unit 52. Terminal9| being connected The former conductor is connected to one side a ofeach of the controller contacts 25 and 35, to the terminal 92 of theunit 52, and to one side of the relay contact lid. The other lineconductor IOI is connected to the terminal SI of the unit 52 and to oneside of each of the solenoids of the valves 41, 49, and 50, as well asto one side of the actuating winding in the relay 5i. The free side ofsaid winding is connected by means of a conductor M2 to the free side ofthe controller contacts 25. The free side of the controller contacts 33is connected by means of a conductor I03 to the terminal 90 of the unit52. Terminal 33 of the unit 52 is connected by a conductor I00 to thefree side of solenoid valve 50 and also to one side of relay contacts5Ic. Terminal 94 of said unit is connected by a conductor I05 to oneside of relay contacts 5Ib, whose free side is connected by means of aconductor I05 to the free side of solenoid 43. The free sides orcontacts Ila and through conductor IOI to one side of the source ofelectric power supply, it follows that any connection of terminal 00 toconductor I00, representing the other side of said source, will causethe interrupter motor 05 to operate, and that during such intervals asthe interrupter contacts 89 are closed, the timer motor 30 and thesolenoid 53 will also be energized. So long as the plate 50 and thecontact elements carried thereby remain in the position of repose, asshown in Fig. 2, terminal 04 will be connected through relay contacts58b to conductor 85, and thus to line conductor I00. Assume for themoment the interrupter contacts 89 to be closed, and the terminal 30placed in connection with the line conductor I00, energizing thesolenoid 53 and the motor 50. The plunger 54, with the stem 55 andelements carried thereby, will assume their upper most position.Contacts 51a and 59a will be closed and contact 58a opened. At the sametime, the spring 61 will be compressed between the collar 55 and theabutment 55, urging the plate and contact elements carried therebytoward the uppermost position, such motion being prevented, however, bythe engagement of the latch member 59 with the detent 68. Also, the worm02 will be brought into operative engagement with the worm wheel 15, andthe motor will cause the same to be driven in such a sense that the pinI5 progressing in a counter-clockwise sense about the axis of rotationof the worm wheel I5 and of the latching member 69, will approach thelatter, and, if the motion be not interrupted, will, at the terminationof a time interval predetermined by the setting of said pin, engage thelatch. causing the same to release the detent 55. Upon release of thedetent 65, the plate 50 will be free I to respond to the upwardinfluence of the spring 51, and will instantaneously assume its topmostposition, as limited by the slots GI whereupon the condition of thethree sets of contacts 51a, 50a, and 53a will be restored to thatexisting when the winding 53 is not energized.

Consideration may now be given to the performance of the apparatushereinbefore set forth, when subjected to normal conditions ofoperation. Upon the line conductors I00 and NI being energized from asuitable source of electrical supply, and air pressure from the source44 being applied to the conduit 43, the normal disposition of theseveral elements of the control system will be as follows:

As the electrical resistance of the air initially in the 'digesteroutlet I4 is relatively high, the pointer 36 of the instrument 35 willassume a position where the contacts 38 will be open, and where theassociated control element 50 will maintain an appreciable regulatedcontrol pressure in the conduit 45. At the same time, the temperature towhich the bulb 2i is exposed being relatively low, the contacts 25 inthe temperature-responsive instrument 20 will stand open, leavingde-energized the winding of the relay 5i, so that the contacts Sid and5") will stand open and the contact 5Ic closed. The contacts 38 beingopen, the solenoid 53 and the motor 00 in the timing unit 52 will bede-energized, so that the several elements of said unit will rest in thepositions indicated in Fig. 2, the contacts 584: standing closed, thecontacts 51a and 58:; standing open, and the contacts 88 being eitheropen or closed, according to the position in which the cam 88, driven bythe interrupter motor 85, may previously have come to rest.

The contacts 51a in the timing unit being open, the actuating winding ofthe solenoid valve 58 will remain de-energized, and that valve willstand in such a position that no air pressure is applied to theactuating element of reverse-acting valve i 8, so that the latter willstand closed. As said contact 51a, and also contact m 01 the relay 5i,are open, the winding of the reverseacting solenoid valve 41 will remainde-energized without respect to the position of relay contacts 5lc, sothat said solenoid valve will stand open, applying to the reverse-actinggas-of! valve l5 such pressure as exists in the conduit 45, to maintainit in a correspondingly open position.

The winding of the solenoid valve 49 being deenergized, since contact5lb is open, said valve will stand in a position to maintaincommunication between conduits 48 and 43, whereby to apply supply airpressure to the direct-acting gasoff valve i6 and maintain the same in aclosed position.

Upon the digester ill being charged, and steam admitted through theconduit i3, a reaction is set up in which there are given off certaingaseous products which tend to accumulate in the upper part of thedigester, and will be continuously vented through the valve l5. So longas the digester blows ofi air from its upper portion, the highelectrical resistivity of the escaping gases will cause the controllingelements of the instrument 35 to be maintained in the conditiondescribed. and the free venting of the eifluent to be continued.

As the non-aqueous gaseous products of chemical reaction within thedigester make their appearance in the vent pipe i4, their relatively lowresistance, corresponding to an unstable condition of operation, willcause the control element 40 of the instrument 35 to assume its normalfunction of so regulating the pressure of air admitted to the conduit 45and through the valve 41 to the gas-01f valve l5 that with a decrease ofresistivity said valve will approach a closed position, and, with anincrease in resistivity, an open position. Thus, there will beestablished a control function eminently suited, as hereinbefore pointedout, to the operation of the digester under the so-called unstablecondition Upon the occurrence of wet gas, due to foaming," or to anycondition causing a portion of the liquid contents of the digester to becarried into the conduit [4, said liquid will provide between theelectrodes 30 and 3| a path of such relatively low electrical resistancethat the pointer 36 in the measuring instrument 35 will rapidly approachthe lowest part of its operating range, causing the contacts 38 to be brdged by the contact element 39, thereby providing a connection betweenthe line conductor Hill and the conductor I03 whereby to energize theinterrupter motor 85 in the timing device 52. As the motor 85 operates,its contacts 89 will be alternately closed and opened; and upon thefirst closing of said contacts 89 subsequent to the closing of contacts38 the relay winding 53 and also the motor 80 will be energized and theworm 82 brought into operative engagement with the worm wheel 15,whereby the latter, under influence of motion l2 transmitted from themotor 88, will be rotated at a predetermined velocity in acounterclockwise sense as seen in the drawing, causing the pin 18 to besteadily advanced toward a position of engagement with the latch member58 to release the same from the detent 48.

While the upward movement of the plunger 84 and the rod or stem 55,carrying the collar 65 will cause a corresponding upward force to betransmitted through the spring 61 to the abutment portion 65, therestraining action 0! the latch 69 upon the detent 68 will inhibitupward displacement of the plate 88, so that the contact elements 51acarried thereby will be bridged by the coacting contact element 51, andthe con-' tact elements 58a by the contact element 5!, while upwarddisplacement of the contact element 58 with respect to coacting contactelements 58a will cause the latter to be electrically separated and thecircuit therebetween interrupted.

Closing of the contacts 58a will provide for the winding 53 anenergizing circuit alternative to that through the contacts 38 in theinstrument 35, so that, after initial energlzation of the solenoidthrough said contacts in series with the interrupter contacts 88, boththe solenoid 53 and the motor will remain energized irrespective of thepositions of said two sets of contacts. As the contacts 5Ib are open,the opening of contacts 58a in series therewith will produce no resuit,and the solenoid valve 48 will remain deenergized, so that the gas-offvalve IE will be held closed by air pressure applied through the conduit48 as hereinbefore set forth. Closing of the contacts 51a will cause thesolenoid valve 50 to be energized, applying line pressure to thereverse-acting valve i8, opening the same, and admitting steam throughthe pipe H to the conduit I4 to blow the contents thereof back into thedigester, purging said conduit, and also cleaning the electrodes 30 and8|. Closing of the contacts 51a will also cause solenoid valve 41 to beenergized through the normally closed relay contacts He, thus exhaustingthe conduit 45, whereby to close the reverse-acting valve i5, so thatall the steam admitted through the valve i8 will pass into the digester,instead of being vented to the atmosphere.

Duration of the purging interval is determined by the time required forthe pin 16 on the wormwheel 15 to travel from its initial location to aposition 01' engagement with the latch 69 to release the latter from thedetent 68. This interval is pre-established more or less empirically,and may readily be changed to suit specific operating conditions. Afterthe motor 80 has run for a time corresponding to this interval, thedetent 68 is released, the plate 60, together with the several contactscarried thereby, under the influence of the spring 61, will be forcedabruptly upward, causing the contacts to assume relative positionscorresponding to a tie-energized state of the soleniod 53, whereupon thepurging cycle will be terminated and the regulation of the digestercaused to revert to that existing prior to development of the wet-gascondition.

If, during the established time interval of the purging cycle, thewet-gas conditions in the conduit [4 have been eliminated, theresistance between the electrodes 303| will have increased suillcientlyto cause the contacts 38 to be opened; and, separation of the contacts58a having opened the locking circuit of the winding 53, that coil, aswell as the motor 80, will at once be de-energized, and the contactelements carried by the stem 55 reset to their normal positions. At thesame time, 'corpression on the spring being relieved, the late 50 andcontact elements borne thereby will follow the plunger 54 to the normalposition, while the latch, being restored by action of the weight I! toits position of rest, will re-engage the detent 58, preparing the timingrelay for its next cycle of operation.

Should it be that the release of the timing relay finds that theabnormal condition has not been relieved, or that the electrodes havenot been cleaned, the contacts 38 will remain closed, so that opening ofthe locking circuit through the contacts 59a will not allow the relay tobecome de-energized. The reversion to normal regulating conditions, asestablished by the upward shift of the plate 50 and contact elementsborne thereby, will then be but momentary; and upon the first opening ofthe contacts 89 by action of the cam 86, the relay 53 will bede-energized for a sufilcient time to permit the plunger 54 and the stem55 to reset to their lowest position, disengaging the worm 82 from theworm-wheel 15, and allowing the latter to reset to its normal positionof rest with the stop 11 in engagement with the abutment l8, and thelatch 69 again to engage the detent 68 upon the plate 60. If thecontacts 38 have not separated, the motor 85 will continue in operation;and, upon the first reclosing of the contacts 89, the purging cycle willbe established, and repeated as hereinbefore described. This repetitionof the purging cycle will be repeated indefinitely until the resistancebetween the electrodes till-3| rises above the value corresponding towet gas in the conduit l4, whereupon the valve l8 will be closed and thesystem will revert to normal control.

The manner of initiation of the purgingcycle and the restoration ofnormal control having been made clear, it may now be assumed that thecook" has processed sufiiciently far that a stable" condition isestablished. This is refiected in an increase in temperature of thevented gases as measured by the instrument 29; and as said temperatureattains an empirically determined value established by the setting ofthe contact arm 24, the electrical contacts 25 will be bridged by thecontact piece 22, and controlling electrical circuits correspondinglyafiected. Closing of the contacts 25 will provide an electrical circuitfrom the line conductor I through the conductor I02 to one side of thewinding of the relay 5|, whose other side is permanently connected tothe line conductor IOI, thus causing said relay to be energized, wherebycontacts 5| 0. and 5th will be closed and contact 5lc opened. Saidcontact 5lc being in series with the now-open contacts 51a in the timingrelay, its opening consequent upon energization of the relay 5| will atthe moment produce no effect. The closing of contacts 5|a, however, willtransfer connection of the solenoid valve 41 from one in which the opencontacts 51a left the circuit incomplete to one in which sa d valve 41will be directly energized from the line conductors I00 and NH, wherebysaid valve will vent the conduit 46 to the atmosphere, removing airpressure from the operating element of the reverse-acting gas-off valvel5, causing the same to revert to a closed position. At the same time,energization of the three-way solenoid valve 49 through contacts 5") inseries with normally closed contacts We will cause said valve to assumea position where the operating element of direct-acting gas-oi! valve15, instead of being held closed by supply air pressure from the conduit43, will be placed under regulation of air received from the controlelement 40 through the conduit 45. Control of the escaping gases willthus be transferred from the reverse-acting valve l5 to thedirect-acting valve IS, with a corresponding reversal in the sense ofregulation in relation to the resistivity of said gases. Consequently,with an increase in resistivity of the effluent, the gas-off valve [5will be moved toward closed position, and vice versa. This provides thetype of performance which, as hereinbefore pointed out, is best suitedto operation under the so-called stable condition; and this will becontinued so long as the temperature measured by the instrument 20 doesnot fall below the value for which the contacts in that instrument areset.

In the event of wet gas appearing during stable control, the resultinglow resistance between the electrodes 303I, as hereinbefore set forth,will cause the instrument 35 to close its contacts 38, actuating thet.ming and interrupter unit 52, and initiating a purging cycle, aspreviously described. Opening of the relay contacts 58a will cause thesolenoid valve 49 to be de-energized, providing direct connectionbetween conduits 48 and 43, whereby to supply pressure to the operatingelement of the directacting valve l6, causing said valve to assume aclosed position, shutting off escape of gases from the dlgester. Closingof the contacts 51a. will cause the solenoid valve 50 to be energized,applying pressure from the supply line to the operating element ofreverse-acting valve I8, opening the same, admitting steam to purge thegasoff main [4 and clean the electrodes 30 and 3|; and the purging cyclewill thenceforth be carried out and terminated in a manner identical tothat already set forth for operation of the digester under unstableconditions.

The terms and expressions which I have employed are used as terms ofdescription and not of limitation, and I have no intention, in the useof such terms and expressions, of excluding an equivalents of thefeatures shown and described or portions thereof, but recognize thatvarious modifications are possible with n the scope of the inventionclaimed.

I claim:

1. In apparatus for regulating the venting of effluent from a processingcontainer evolving a normally gaseous ellluent, means responsive to theelectrical resistivity of said gaseous eilluent, means for controllingthe venting of said efiiuent, and means controlled by the firstmentioned means for impressing a modulated performance upon saidcontrolling means in accordance with the resistivity of said gaseouseiiiuent.

2. In apparatus for regulating the venting of effluent from a processingcontainer evolving a normally gaseous eiliuent, means responsive to theelectrical resistivity of said gaseous eflluent, means for controllingthe venting of said effluent, and means controlled by the firstmentioned means for operating said controlling means to increase saidventing in response to increase in resistivitiy of said gaseous eiliuentand to decrease said venting in response to decrease in saidresistivity.

3. In apparatus for regulating the venting of eflluent from a processingcontainer evolving a normally gaseous eflluent, means responsive to theelectrical resistivity of said gaseous eilluent, means for controllingthe venting of said effluent, and means controlled by the firstmentioned means for operating said controlling means to increase saidventing in response to increase in resistivity of said gaseous eiiiuentand to decrease said venting in response to decrease in saidresistivity, and means responsive to another characteristic of saideilluent for modifying the action of said regulating means in one sensewith a change of said characteristic in one direction and rendering saidaction'eifective in its original sense with a change of saidcharacteristic in the other direction.

4. In apparatus for regulating the venting of eiliuent from a processingcontainer evolving a normally gaseous effluent, means for determiningthe electrical resistivity of said effluent, means for determining thetemperature of said eiliuent, control means actuated in response tochanges in said electrical resistivity to regulate the flow of saideffluent, and further control means, actuat ed in response to changes insaid temperature, to modify the performance of said first control meansin one sense with a change of said temperature beyond a predeterminedvalue in one direction, and to render said performance eifective in itsoriginal sense with a change of said temperature beyond said value inthe other direction.

5. In apparatus for regulating the venting of eiliuent from a processingcontainer evolving a normally gaseous effluent, means for determiningthe electrical resistivity of said eilluent, means for determining thetemperature of said eiliuent, control means adapted to increase theescape of eilluent with an increase in said electrical resistivity andto decrease said escape with a decrease in said resistivity, togetherwith further control means responsive to said temperaturedeterminingmeans and adapted upon an increase of said temperature above apredetermined value to reverse the characteristic performance of saidfirst control means, and upon a decrease of said temperature below saidpre determined value to restore said performance to its originalcharacteristic.

6. In apparatus for regulating the venting oi' eiliuent through aconduit extending from a processing container evolving a normallygaseous effluent, means for determining the electrical resistivity ofsaid etlluent, means for determining the temperature of said enluent,first and second valves adapted alternatively to discharge eiliuent fromsaid conduit, control means adapted alternatively to maintain saidsecond valve in a closed condition while regulating said first valve ina sense to be opened with an increase in said resistivity, or tomaintain said first valve in a closed position while regulating saidsecond valve in a sense to be closed with an increase in saidresistivity, and further control means subject to saidtemperature-determining means and influencing said first-named controlmeans whereby to select between said two characteristic performancesaccording to the temperature of said effluent.

7. In apparatus for regulating the venting of eiliuent from a processingcontainer evolving a normally gaseous eiiiuent, means responsive to theelectrical resistivity of said gaseous eiliuent, valve means forcontrolling the venting of said eiiiuent, and means controlled by thefirst mentioned means for regulating said valve means to control saidventing in accordance with variations in electrical resistivity oi saidgaseous effluent.

8. In apparatus for regulating the venting of efliuent from a processingcontainer evolving a normally gaseous eiiluent, means responsive to theelectrical resistivity of said gaseous eilluent, a valve for controlhngthe venting of said eiiluent, and means controlled by the firstmentioned means for operating said valve to increase said venting as theresistivity of said gaseous emuent increases and for operating saidvalve to decrease said venting as said resistivity decreases. 9. Inapparatus for regulating the venting of eiiluent from a processingcontainer evolving a normally gaseous eiiluent, means responsive to theelectrical resistivity of said gaseous eiiluent, a valve for controllingthe venting of said eiiiuent, means controlled by the first mentionedmeans for operating said valve to increase said venting as theresistivity of said gaseous emuent increases and for operating saidvalve to decrease said venting as said resistivity decreases, a secondvalve, control means for*said second valve for maintaining the latterclosed while the first valve is in operation, means responsive to thetemperature of said effluent, and control means including connectionsfrom said temperature-responsive means activated upon a change in saidtemperature beyond a predetermined value in one direction to causeclosing of said first valve and to place said control means for saidsecond valve under control of said resistivity-responsive means foroperating said second valve in the opening direction as said resistivitydecreases and in the closing direction as said resistivity increasesand, upon a change in said temperature beyond said value in the oppositedirection, activated to restore the performance of said valves to theoriginal characteristic.

10. In apparatus for regulating the venting of eiliuent through aconduit extending from a. processing container evolving a normallygaseous eifluent; means for determining the resistance of a portion ofsaid etlluent, means for determining the temperature of said eiiluent,control means actuated by said resistance-determining means andoperative thereby when the measured resistance is of the ordercorresponding to a gaseous condition of said eilluent, to permit thecscape of said eilluent to a degree increasing with the measuredresistance thereof, and subject to said temperature-determining meansupon the increase of said temperature above a predetermined value toreverse its characteristic performance and restrict the escape of saideiliuent to a degree increasing with the measured resistance thereof andupon decrease of said temperature below said value to restore theperformance of said control means to its original characteristic.

11. In apparatus for regulating the venting of eflluent from aprocessing container evolving a normally gaseous effluent, meansresponsive to the electrical resistivity of said gaseous emuent. meansfor controlling the venting of said eiiiuent, and means controlled bythe first mentioned means for operating said controlling means todecrease the venting as said resistivity increases.

12. In apparatus for regulating the venting of emuent from a conduit ofa processing container evolving a normally gaseous ei'iluent, meansresponsive to the resistivity of said gaseous eifluent. means forcontrolling the extent of venting of said eflluent, means controlled bysaid resistivityresponsive means for operating said vent-controllingmeans to vary the extent of venting of said effluent, and means renderedactive upon said resistivity attaining a predetermined low valuecorresponding to the presence of liquid in said effluent for causingclosure of said vent-controlling means irrespective of the action ofsaid operating means and for re-opening said ventcontrolling means uponsaid resistivity exceeding said low value.

13. In apparatus for regulating the venting of effluent from a conduitof a processing container evolving a normally gaseous effluent, meansresponsive to the resistivity of said gaseous effluent, means forcontrolling the extent of venting of said effluent, means controlled bysaid resistivityresponsive means for operating said vent-controllingmeans to vary the extent of venting of said effluent, means for admittina flushing fluid to said conduit to flush the contents thereof, andmeans rendered active upon the attainment of a predetermined lowresistivity value corresponding to the presence-of liquid in saideffluent for causing opening of said admitting means and closure of saidvent controlling means, and for causing closing of said admitting meansand opening of said vent controlling means upon said resistivityexceeding said low value.

14. In apparatus for regulating the venting of effluent from a conduitof a processing container evolving a normally gaseous effluent, meansresponsive to the resistivity of said gaseous effluent, means forcontrolling the extent of venting of said effluent, means controlled bysaid resistivityresponsive means for operating said vent-controllingmeans to vary the extent of venting of said effluent, means responsiveto a predetermined low resistivity corresponding to the presence ofliquid in said effluent for causing closure of said vent-controllingmeans irrespective of the action of said operating i eans timing means,and means controlled by said timing means for causlng restoration ofsaid vent-controlling means to the action of said operating means.

15. In apparatus for regulating the venting of effluent from a conduitof a processing container evolving a normally gaseous effluent, meansresponsive to the resistivity of said gaseous effluent, means forcontrolling the extent of venting of said effluent, means controlled bysaid res stivityresponsive means for operating said vent-controllingmeans to vary the extent of venting of said effluent, means foradmitting a flushing fluid to said conduit to flush the contertsthereof, means responsive ,to a predetermined low resistivitycorresponding to the presence of liquid in said effluent for causingclosure of said vent-controlling means irrespective of said operatingmeans and for causing opening of said admitting means, timing means, andmeans controlled by said timing means for causing closing of saidadmitting means and restoration of said vent-controlling means to theaction of said operating means.

16. In apparatus for regulating the venting of effluent from a conduitof a processing container evolving a normally gaseous effluent, meansresponsive to the resistivity of said gaseous effluent, means forcontrolling the extent of venting of said effluent, means controlled bysaid resistivityresponsive means for operating said vent-controllingmeans to vary the extent of venting of said effluent, means foradmitting a flushing fluid to said conduit to flush the contentsthereof, and means rendered active upon a predetermined low value ofeffluent resistivity corresponding to the presence of liquid therein foropening said admitting means and for closing said admitting means uponsaid resistivity exceeding said low value.

17. In apparatus for regulating the venting of effluent from a conduitof a processing container evolvinga normally gaseous effluent, meansresponsive to the resistivity of said gaseous effluent, means forcontrolling the extent of venting of said effluent, means controlled bysaid resistivityresponsive means for operating said vent-controllingmeans to vary the extent of venting of said effluent, means foradmitting a flushing fluid to said conduit to flush the contentsthereof, means responsive to a predetermined low effluent resistivitycorresponding to the presence of liquid therein for opening saidadmitting means, timing means for maintaining said admitting means openfor a predetermined time interval, and means operative upon terminationof said interval to effect closing of said admitting means.

18. In apparatus for regulating the venting of effluent from aprocessing container evolving a normally gaseous effluent, means forcontinuously determining the electrical resistivity of said gaseouseffluent, means for controlling the venting of said effluent, and meanscontrolled by the first-mentioned means for operating said controllingmeans in accordance with said resistivity.

19. In apparatus for regulating the venting of effluent from aprocessing container evolving a normally gaseous effluent, means forcontinuously determining the electrical resistivity of said 7 effluent,means for continuously determining the temperature of said effluent,control means actuated in response to changes in said electricalresistivity to regulate the flow of said effluent, and further controlmeans actuated in response to changes in said temperature beyond apredetermined value in one direction for modifying the performance ofthe first-mentioned control means in one sense, and in response tochanges of said temperature beyond said value in the opposite directionto modify said performance in another sense.

20. In apparatus for regulating the venting of effluent from aprocessing container evolving a normally gaseous effluent, meansresponsive to the electrical resistivity of said gaseous effluent,

means for controlling the venting of said effluent, and pneumatic meanscontrolled by the firstmentioned means for impressing a .modulatedperformance upon said controlling means in accordance with theresistivity of said gaseous effluent.

21. In apparatus for regulating the venting of effluent from aprocessing container evolving a normally gaseous effluent, means forcontinuously determining the electrical resistivity of said effluent,means for controlling the venting of said effluent, and pneumatic meanscontrolled by the first-mentioned means for operating said controllingmeans in accordance with said resistivity.

22. In apparatus for regulating the venting of effluent through aconduit extending from a processing container evolving a normallygaseous fluent, to permit the escape of said eiiiuentto an extentincreasing with the measured resistance thereof, and subject to saidtemperature-determining means upon the increase of said temperatureabove a predetermined value to reverse its characteristic performanceand restrict the escape of said eiiluent to a degree decreasing with themeasured resistivity thereof and upon decrease of said temperature belowsaid value to restore the performance of said control means to itsoriginal characteristic.

23. In apparatus for regulating the venting of efliuent from aprocessing container evolving a normally gaseous efliuent, means fordetermining the electrical resistivity of said efliuent, means fordetermining the temperature of said eiiluent, control means forproviding a modulated regulation upon the flow of said etlluent inaccordance with the resistivity thereof in its gaseous condition,further control means actuated in response to changes in saidtemperature beyond a predetermined value in one direction to modify theperformance of said first control means in one sense, and in response tochanges of said temperature beyond said value in the opposite directionto modify said performance in another sense, means for admitting aflushing fluid to a portion of said container to flush eilluent materialtherefrom, and means brought into action in response to a predeterminedlow resistivity of said eiiluent for actuating said admitting means.

24. In apparatus for regulating the venting of emuent from a conduit ofa processing container evolving a normally gaseous eiliuent, means forcontrolling the extent of venting of said emuent,

means for determining the electrical resistivity of said eiiiuent, meanscontrolled by said resistivity-responsive means for operating saidventcontrolling means to vary the extent of venting of said eiiiuent,means for admitting a flushing fluid to said conduit to flush thecontents thereof, means responsive to a predetermined low resistivitycorresponding to the presence of liquid in said eiiiuent for causingclosure of said ventcontrolling means irrespective of said operatingmeans and for causing opening of said admitting means, timing means,means controlled by said timing means for causing closing of saidadmitting means and restoration of said vent controlling means to theaction of said operating means, means responsive to the temperature 0!said emuent, and means controlled by said temperature-responsive meansfor reversing the action of said vent controlling means, the directionof said reversal depending upon the sense of change in said temperature.

ROBERT D. COWHERD.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,145,509 Pike July 6, 19151,388,613 Simsohn Aug. 23, 1921 2,395,357 'Irawick Feb. 19, 19462,396,308 Williams Mar. 12, 1946 2,411,986 Cowherd Dec. 3, 1946

